Protozoan Grazing and the Size–Activity Structure of Limnetic Bacterial Communities

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We measured bacterial growth rates by labelled thymidine incorporation, grazing loss rates with fluorescent latex particles, and bacterial cell size and abundance within narrow size-fractions of freshwater bacterioplankton. Contrary to the predictions of standard allometric relationships, the smallest bacteria showed the lowest incorporation rate per cell and per unit DNA content. Cells trapped by 1- and 5-μm filters grew the fastest and were responsible for 86% of detected thymidine biosynthesis. Grazing studies in six lakes showed that grazing pressure from flagellated protozoans and mixotrophic algae on large bacterial cells was probably 2–40 times more intense than that on the smallest cells. We suggest that reduced grazing pressure on the smallest bacteria allows them to dominate numerically, despite their slower growth.